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[Photographic  Eeproduction   (Actual  Size)   of  the  Original  Illustration  in  Helm- 
holtz's    "Beschreihung   eines   Angenspiegels."] 


THE  DESCRIPTION 

OF  AN 

OPHTHALMOSCOPE 

BEING  AN  ENGLISH  TRANSLATION  OF 

Von  HELMHOLTZ'S 
"Beschreibung  eines  Augenspiegels" 

(BERLIN,  1851) 
BY 

Thomas  Hall  Shastid,  A.B.,  A.M.,  M.D.,  LL.B.,  F.A.C.S. 

SUPERIOR,  WISCONSIN 
And  the  first  translation  of  this  classic  into  any  language 


Chicago 

CLEVELAND  PRESS 

1916 


J 


Copyright  1916 

By 

THOS.  HALL  SHASTID 


All  rights  reserved. 


Sebication 


It  is  fitting  that  this,  the  first  translation  into  any  language  of 

Helmholtz's  "Beschreibung  eincs  Augenspiegels," 

should  be  inscribed  to 

3Br.  Casftp  a.  aaioob 

Ophthalmologist  and  almost  universal  scholar,  who  has  contributed 
so  much  to  our  knowledge  of  the  visual  apparatus,  and  who,  in  addi- 
tion, has  so  indefatigably  gathered  together  and  preserved  (as  in  the 
American  Encyclopedia  of  Ophthalmology  and  works  of  a  similar  char- 
acter) the  comprehensive  literature  of  our  subject.  Because  of  a 
friendship  lasting  now  these  many  years,  he  consents  to  waive  the 
imperfections  of  my  rendering,  and  to  accept  this  very  slight 
performance — which  is  offered  merely  as  a  little  token  of  a  high  regard. 
May  the  fraternal  reader  be  as  lenient  toward  those  imperfections  as 
is  he,  and — I  may  add — as  appreciative  (for  many  are  not)  of  the 
timeless,  the  deathless  character  of  the  little  document  which  I  have 
had  the  temerity  to  try  to  translate.  — T.  H.  S. 


Edition   limited   to  500   copies. 
For    private    distribution    only. 


DESCRIPTION 


OP   AN 


OPHTHALMOSCOPE 

FOR  THE  INVESTIGATION 
OF  THE  RETINA  IN  THE  LIVING  EYE 


BY 


H.  HELMHOLTZ, 

PROFESSOR  OF  PHYSIOLOGY  AT  THE  UNIVERSITY  OP   KOENIGSBERG. 


BERLIN 

THE  A.  FORSTNER  PUBLISHING  HOUSE 

(p.  jeanrenaud) 

1851 

[A  translation  of  von  Helmholtz  's  title-page,  treated  typographically  as  nearly 
like  the  original  as  possible.] 


THE  OPHTHALMOSCOPE 


The  present  treatise  contains  the  description  of  an  optical  instrument, 
by  which  it  is  possible  in  the  living  to  see  and  recognize  exactly  the 
retina  itself  and  the  images  of  luminous  objects  which  are  cast  upon  it." 


*  Even  tho  ancients,  as  a  matter  of  course,  had  noticed  that  the  ejes  of  cer- 
tain animals  are  brilliant  in  the  dark.  Thus  Pliny  (Book  XI,  Chap.  55):  "The 
eyes  of  animals  that  see  at  night  in  the  dark,  cats  for  example,  are  shining  and 
radiant,  so  much  so  that  we  cannot  bear  to  gaze  upon  them;  those  of  the  she- 
goat,  too,  and  the  wolf  are  resplendent,  and  emit  a  light  like  fire."  Pliny  did 
not,  however,  attempt  to  explain  the  phenomenon. 

In  1704,  Jean  Mery,  of  Paris,  performed  his  famous  experiment  with  a  cat. 
Having  immersed  the  animal  in  water,  he  first  observed  that  the  pupil  dilated 
(as  a  result  of  suspended  respiration)  and  then  he  beheld  in  all  its  glory  the 
fundus  of  the  animal 's  eye — the  entrance  of  the  optic  nerve  and  all  the  colors 
and  vessels  of  the  choroid.  Mery  understood  quite  well  enough  that  something 
more  than  mere  pupillary  dilatation  was  necessary  to  account  for  the  possibility 
of  observing  the  fundus  of  the  eye  when  the  eye  was  under  water.  His  ex- 
planation, however,  of  the  "something  more"  was  wholly  erroneous.  He 
believed,  that  is  to  say,  that  the  view  of  the  fundus  was  rendered  possible  by 
the  water's  filling  up  a  multitude  of  tiny  " unevennesses "  on  the  anterior  sur- 
face of  the  cornea.  Five  years  later,  de  la  Hire  stepped  forward  with  the  cor- 
rect explanation.  According  to  him,  the  water  obviated  the  refraction  of  light 
by  the  cornea,  so  that  all  rays  leaving  a  given  point  upon  the  fundus  emerged 
from  tho  eye  not  as  parallel,  but  as  divergent,  rays.  De  la  Hire  also  observed, 
incidentally,  that  the  disturbing  light-reflexes  proceeding  from  a  cornea  in  aero 
are  done  away  with  by  the  water. 

In  1796  Fermin  observed  a  certain  luminosity  in  the  pupils  of  an  Ethiopian 
albino.  In  1816  Scarpa  remarked  upon  a  similar  phenomenon  in  a  certain  dis- 
ease of  the  fundus,  and,  one  year  later,  Beer  described  the  same  condition  fully, 
inventing  therefor  the  expression  "amaurotic  cat's  eye" — a  term  which  is  still 
in  use.  In  1836  Hasenstein  first  produced  a  factitious  luminosity  by  compress- 
ing the  eveball  backward — making  the  eye,  in  fact,  artificially  hypermetropic. 
In  1847  Babbage,  an  English  mathematician,  exhibited  to  Wharton  .Tones,  the  dis- 
tinguished London  oculist,  the  model  of  an  instrument  invented  by  him  for  the  pur- 
pose of  examininsr  the  interior  of  the  eye.  It  consisted  of  a  small  plane  glass 
mirror,  from  which  a  portion  of  the  silvering  had  been  removed.  This  device, 
however,  was  first  made  known  to  the  world  in  1854,  by  Wharton  Jones  (Brit,  and 
For.  Medico  Chir.  Beview,  Oct.,  1854).  The  services  of  Briicke  Hn  1845,  published 
in  1847)  and  of  C'umming  (in  1846)  are  adverted  to  herein  by  Helmholtz. 

Tho  earliest  reception  of  the  ophthalmoscope  is  decidedly  interesting.  Thus, 
to  quote  from  Koenigsberger,  "Hermann  von  Ilelmholtc,"  (1906)  p.  74:  "The 
ophthalmoscope  was,  however,  some  time  in  making  its  way,  on  account  of  the 
mathematical  and  physical  knowledge  presupposed  by  the  '  Description  of  an  Oph- 
thalmoncope  for  the  Inve.itination  of  the  Retina  in  the  Livinrj  Eye,'  published  in 
the  autumn  of  1851,  and  people  were  at  first  very  shy  of  employing  it.  One  dis- 
tinguished surgical  colleague  told  Ilebnhnltz  he  should  never  use  the  instrument — 
it  wo\ild  be  too  dangerous  to  admit  the  naked  light  into  a  diseased  eve;  another 
was  of  opinion  that  the  mirror  might  be  of  service  to  oculists  with  defective  eye- 
sight— he  himself  had  good  eyes  and  wanted  none  of  it." — (T.  H.  S.) 

7 


THE  OPHTHALMOSCOPE 

The  instrument  has,  for  this  purpose,  two  different  problems  to  solve. 
First,  everything  which  we  can  see  of  the  backgi-ound  of  the  uninjured 
eye  appears  to  us  absolutely  dark.  The  cause  of  this  lies,  as  I  will  show, 
in  the  light-refracting  media  of  the  eye,  which,  under  ordinary  circum- 
stances, hinder  us  from  seeing  illuminated  parts  of  the  retina  behind 
the  pupil.  Therefore,  the  first  question  is  to  discover  a  means  of  il- 
lumination whereby  exactly  that  portion  of  the  retina  on  which  we  gaze 
through  the  pupil  may  be  adequately  lighted.  Secondly,  we  view  the 
background  of  the  eye  only  through  the  light-refracting  media.  These, 
however,  cast  images  of  the  retinal  objects,  which,  in  general,  do  not  lie 
for  the  observer  within  the  limits  of  plain  vision.  We  need,  therefore, 
together  with  a  proper  procedure  for  illumination,  also  further  optical 
expedients  which  will  render  possible  for  the  observing  eye  a  correct 
accommodation  for  the  objects  which  it  should  see. 

I.  Illumination. 

In  order  to  be  able  to  find  the  essential  conditions  for  the  method  of 
illumination,  we  must  first  of  all  make  clear  to  ourselves  why,  as  a  rule, 
the  ground  of  the  eye  behind  the  pupil  appears  to  us  to  be  of  so  deep 
a  black. 

The  cause  of  this  is  not  the  condition  of  the  pigment  of  the  chorioidea; 
for,  if  the  pigment  layer  absorbed  the  light  which  falls  upon  it  even 
more  completely  than  any  other  known  black  substance,  still,  there  lie 
before  the  chorioidea  parts  which  can  reflect  a  quantity  of  light  suffi- 
cient to  render  them  visible.  That  is  true,  first  of  all,  of  the  substance 
of  the  x-etina,  which,  to  be  sure,  in  the  recent  condition,  is  very  trans- 
parent, and  marks  itself  off  but  little  against  the  dark  pigmentary 
backgi'ound ;  to  a  much  higher  degree,  however,  it  is  true  of  the  blood- 
vessels of  this  membrane,  whose  tiny  stems  carry  blood  enough  to 
exliibit  a  strongly  red  hue.  Finally,  there  appears,  even  in  the  fundus 
of  the  eye,  a  shining  white  spot,  namely,  the  place  of  entrance  of  the 
optic  nerve,  on  which  no  pigment  at  all  lies,  and  which,  therefore, 
reflects  all  the  light  that  falls  upon  it.  And  yet  we  observe,  under 
ordinary  circumstances,  behind  the  pupil  of  the  living  eye,  not  the 
slightest  trace  of  the  red  color  of  the  blood,  nor  of  the  white  color  of 
the  optic  nerve. 

It  can  be  shown  much  better  by  a  simple  experiment,  that  not  the 
color  of  the  background,  but  only  the  refraction  of  the  light  in  the  ocu- 
lar media,  is  the  cause  of  the  deep  blackening  of  the  pupil.  Let  one  take 
any  kind  of  small  camera  obscura  well  blackened  within,  and  let  him 
bring  to  the  place  where  the  picture  is  produced  an  opaque  white 


THE  OPHTHALMOSCOPE  9 

card,  for  example  one  from  thick  white  drawiug-paper.  Among  other 
kinds  of  camera  may  be  employed  the  ocular  tubes  of  most  microscopes, 
after  the  ocular  glass  has  bceu  removed  therefrom,  and  the  collective 
glass  has  been  inserted.  These  tubes  are,  as  a  rule,  precisely  as  long 
as  the  local  distance  of  the  collective  glass.  If  one  sets  them  with  the 
end  which  contains  the  ocular  upward  upon  the  white  card,  then  they 
form  a  camera  obscura  of  the  kind  we  need.  There  are  thrown,  in 
this  case,  very  bright  images  of  the  surrounding  illuminated  objects, 
on  the  white  card,  and  still  the  interior  of  the  instrument,  w'hen  one 
looks  into  the  lens  in  any  desired  direction,  appears  absolutely  black. 
We  have  here  a  fac  simile  of  the  eye,  where  cornea  and  crystalline  lens 
are  substituted  by  the  objective  lens  of  the  camera,  and  the  retina  by 
a  clear  white  paper  surface,  but  there  occurs  apparently  the  same 
complete  darkness  of  the  internal  space  as  in  the  eye,  as  long  as  the 
paper  surface  lies  precisely  at  the  spot  where  the  tiny  images  of  ex- 
ternal objects  are  produced.  If  one  takes  away  the  convex  glass,  or 
if  one  materially  alters  its  distance  from  the  paper  surface,  there 
appears  to  the  beholder  at  once  the  clear  white  surface  of  the  paper. 

How,  now,  can  the  refraction  of  the  light  produce  the  phenomenon 
described  ?  Let  us  consider  the  course  which  the  rays  of  light  must 
take,  according  to  the  physical  laws  of  the  refraction  of  light  in  the 
eye. 

Let  light  fall  from  a  luminous  point  upon  a  fittingly  adapted  eye, 
concerning  which  we  assume  that  it  is  formed  with  absolute  accuracy, 
that  is,  that  all  the  incident  rays  from  the  point  in  question  concentre 
upon  a  single  point  of  the  retina.  Of  the  light  which,  by  the  ocular 
media,  is  caused  to  converge  upon  this  membrane,  the  greater  part 
is  absorbed  by  the  black  pigment,  while  the  smaller  is  reflected  partly 
by  the  nerve  elements  and  blood-vessels,  partly  by  the  layer  of  rod- 
shaped  corpuscles.  That  which  is  thrown  back  by  the  latter  structures, 
passes,  as  E.  Briicke  has  shown,  back  out  through  the  pupil,  without 
beeonung  scattered  to  any  other  portion  of  the  wall  of  the  eye.  In 
this  way  is  avoided  the  spreading  of  perceptible  quantities  of  dispersed 
light  within  the  eye.  The  reflected  rays,  which,  from  the  point  of  con- 
vergence on  the  retina,  pass  back  out  divergently  to  the  refracting 
surfaces  of  the  eye,  follow  then  precisely  the  same  path,  in  a  reverse 
direction,  by  which  the  incident  rays  of  the  luminous  point  converged 
from  the  refracting  surfaces  of  the  eye  until  they  reached  the  retina. 
From  this  it  follows  that  the  returning  rays,  even  after  they  have 
passed  clear  through  the  refractive  media  and  out  of  the  eye,  must 
coincide  completely  with  the  incident  rays,  mu.st  therefore  finally  all 
betake  themselves  to  the  original  luminous  point. 


10  THE  OPHTHALMOSCOPE 

For,  when  two  rays,  which  pass  through  several  simply  refracting 
media  in  a  reverse  direction,  coincide  in  one  of  the  same  [media]  they 
must  do  the  same  in  all  [the  media] .  On  the  limiting  surfaces  of  the 
medium,  that  is  to  say,  within  which  they  coincide,  the  angle  of  inci- 
dence of  the  outcomiug  rays  is  identical  with  the  angle  of  refraction 
of  those  which  are  entering.  As,  now,  according  to  the  laws  of  refrac- 
tion, the  proportion  of  the  sine  between  the  angle  of  incidence  and  the 
angle  of  refraction  of  the  former,  is  precisely  as  large  as  that  between 
the  angle  of  refraction  and  the  angle  of  incidence  of  the  latter,  so 
must  also,  on  the  other  side  of  the  refracting  surface,  the  angle  of 
refraction  of  the  outcoming  and  the  angle  of  incidence  of  the  ingoing, 
rays,  be  equal.  As,  at  the  same  time,  all  these  rays  lie  in  one  plane, 
the  plane  of  refraction,  it  follows  that  they  also  fall  into  one  another 
[coincide]  in  the  second  medium.  In  like  manner  it  follows  further 
for  the  third,  fourth  medium,  and  so  on. 

Let  us  apply  that  to  the  case  where  any  given  system  of  refracting 
surfaces  produces  an  exact  image  of  the  luminous  point  a  at  the  point 
h,  that  is,  where  all  the  rays  which  proceed  from  a  unite  again  in  &, 
then  follows  the  well  known  fact  that  in  this  case,  always,  a  will  be 
the  image  of  b,  if  the  latter  sends  out  rays.  Exactly  upon  the  same 
paths,  that  is  to  saj',  on  which  rays  from  a  proceed  to  b,  they  may 
also  return  from  h  to  a.  If  now  o  is  a  luminous  point  outside  the  eye, 
and  b  its  image,  a  point  on  the  retina,  then  the  ocular  media  will  con- 
centrate the  returning  light  precisely  at  a  into  an  image  of  i.  The 
image  of  the  illuminated  retinal  point  will  coincide  exactly  with  the 
original  point  of  luminosity.  The  same  is  still  valid,  also,  when  we 
have  to  do  not  with  a  luminous  point,  but  with  a  luminous  surface  or 
a  body,  as  soon  as  the  eye  is  adapted  for  its  outlines.  All  the  incident 
light  which  is  thrown  back  can  always  only  return  to  its  place  of 
origin,  and  never  can  proceed  in  any  other  direction. 

From  this  it  follows  that,  without  special  expedients,  we  can  see 
nothing  of  the  illuminated  portion  of  the  retina,  because  we  cannot 
bring  our  eye  into  the  direction  of  the  returning  light  without  at  the 
same  time  cutting  oflF  the  incident  light  absolutely.  To  our  pupil  no 
light  from  the  depths  of  the  other's  e.ye  can  return  which  has  not  pro- 
ceeded from  it  [1.  e.,  our  pupil].  And  as,  in  general,  none  at  all  has 
proceeded  from  our  pupil,  it  sees  in  the  darkness  of  the  other's  eye 
merely  the  reflection  of  its  own  blackness;  only  those  portions  of  the 
retina  become  visible  to  it  on  which  its  own  dark  image  is  copied. 

We  have  until  now  assumed  that  the  observed  eye  furnishes  abso- 
lutely accurate  images.  When  that  is  not  the  case,  the  propositions 
heretofore  laid  down  do  not  hold  strictly  true,  the  returning  light  will 


THE  OPHTHALMOSCOPE  11 

indeed  proceed  to  the  illuminating  body,  but  it  will  also  in  part  pass 
by  tiiat,  and  an  observer  who  approximates  himself  to  the  line  of  direc- 
tion of  the  incident  light,  will  be  able  to  perceive  a  part  of  the  light 
which  is  coming  out.  On  this  fact  are  based  the  methods  of  Gumming 
{Medic.  Chirurg.  Transaotions,  Vol.  29,  p.  2b-i)  and  Briicke  (</.  Mailers 
Archiv.  1»47,  p.  225)  for  observing  the  illumination  of  human  eyes. 
From  what  has  been  said  it  is  manifest  that,  in  this  way,  the  illumina- 
tion must  be  the  greater,  the  less  exactly  the  rays  of  a  luminous  point 
are  concentrated  on  a  point  of  the  retina,  therefore  especially  in  faulty 
adaptation.  Besides,  1  have  convinced  myself  that  one  may  observe 
a  weak  illumination,  according  to  the  method  of  E.  Briicke,  even  in 
eyes  with  good  acuity  and  under  perfect  adaptation  for  the  luminous 
body,  from  which  is  to  be  concluded  that,  under  all  circumstances,  a 
small  quantity  of  incident  light  is  scattered  laterally.  The  cause 
thereof  may  be  inexactness  of  the  eye,  incomplete  transparency  of  its 
refracting  parts,  or  ditiraction  at  the  border  of  the  pupil. 

In  any  case,  the  observer  perceives,  in  this  experiment,  only  a  small 
part  of  the  returning  light,  and  indeed  precisely  that  which  is  irregu- 
larly refracted  and  which  cannot  be  used  for  the  production  of  a 
regular  image.  Some  other  method  is  necessary  for  the  attainment 
of  our  object,  a  method  which  makes  it  possible  to  look  into  the  eye 
not  merely  somewhat,  but  exactly,  in  the  direction  of  the  incident 
light.  The  expedient  for  this  has  already  been  found  in  an  accidental 
observation  by  E.  Briicke.  v.  Erlach,  who  wore  spectacles,  saw,  in- 
deed, the  eyes  of  an  acquaintance  shine,  when  the  acquaintance  saw 
reflected  in  the  lenses  of  the  spectacles  a  light  which  there  was  in  the 
room.  In  this  way,  therefore,  uncovered  lenses  were  employed  as 
illuminating  mirrors,  and  through  these  very  objects  the  observer 
looked  toward  the  observed  eye.  Precisely  the  same  expedient  we 
shall  employ  for  our  purpose,  replacing,  however,  the  spectacle  lenses 
to  advantage  by  well  ground  plane  glasses. 

In  a  darkened  room,  where  only  a  single  source  of  light,  a  well  burn- 
ing lamp  or  an  opening  in  the  window  shutter  for  the  sunlight,  is 
present,  let  one  set  a  small,  plane  glass  plate  in  such  a  way  that  the 
observed  eye  may  perceive  therein  the  mirrored  image  of  the  light, 
without,  however,  its  necessarily  gazing  at  this  mirrored  image  directly. 
From  out  the  anterior  surface  of  the  lens  there  falls,  by  this  arrange- 
ment, light  into  the  observed  eye,  and  through  the  same  glass  at  the 
same  time  the  observer  can  view  the  eye,  without,  while  so  doing, 
being  in  the  least  aware  of  any  light  which  is  being  reflected  from  its 
anterior  surface.  One  sees  that,  in  this  way,  it  becomes  possible  to 
look  into  the  subject's  eye  in  precisely  the  same  direction  as  that  in 


12  THE  OPHTHALMOSCOPE 

which  the  light  I'alls  upou  it.  Under  these  circumstances  the  eye  of 
the  observer  in  fact  receives  light  from  out  the  depths  of  the  other 
eye,  and  sees  its  pupil  apparently  grow  luminous. 

In  Fig.  1,  let  A  be  the  tiame,  C  the  glass  plate,  D  the  observed,  G 
the  observing  eye.  The  light  from  A  which  falls  upon  the  mirroring 
plate,  is  by  that  partly  reflected,  and  the  reflected  part  continues, 
according  to  the  laws  of  catoptrics,  as  if  it  proceeded  from  the  reflected 
image  of  the  flame  at  B.  For  the  observed  eye  this  mirrored  image 
represents  the  place  of  the  luminous  object,  and  upon  its  retina  is 
thrown  an  inverted  and  minified  image.  Moreover,  the  axis  of  this 
eye  can  be  turned  in  any  direction,  say  toward  the  object  H.  Accord- 
ing to  the  already  developed  rules,  the  refractive  media  of  D  cast  the 
image  of  its  retina  and  of  its  retinal  images  again  at  B.  For  B  is  the 
apparently  present  object  for  the  eye  D,  and  the  rays  returning  from 
D  must  proceed  again  to  their  place  of  origin.  On  the  way  from  D 
to  B  this  light  encounters  once  more  the  reflecting  plate,  a  part  is 
reflected  and  returns  to  the  real  flame  A,  another  portion  passes 
through  the  glass  and  strikes  the  eye  of  the  observer  G. 

By  this  arrangement  the  pupil  of  the  eye  D  appears  to  shine  with  a 
red  light,  and  indeed  as  a  rule  more  strongly  than  I  have  seen  it  do 
by  Briicke's  method.  According  to  that  method,  there  contributes 
to  the  illumination  only  the  little  light  which,  in  the  eye,  is  not  com- 
pletely and  regularly  refracted;  according  to  the  method  just  de- 
scribed, on  the  contrary,  the  entire  light,  with  the  exception  of  the 
part  (to  be  sure  not  an  inconsiderable  part)  which  is  lost  by  the  pas- 
sage through  the  reflecting  glass.  Moreover,  the  illumination  is  of 
very  different  strengths,  when  different  portions  of  the  retina  receive 
the  image  of  the  flame.  "When  the  eye  D  turns  in  different  directions, 
the  clear  retinal  image  must  always  remain  in  the  prolongation  of  the 
line  B  D,  and  will  therefore  fall  successively  on  various  portions  of 
the  background.  If  it  falls  on  the  place  of  entrance  of  the  optic 
nerve,  then  the  most  of  the  light  is  reflected,  the  pupil  lights  up  with 
a  strong  yellowish  white,  almost  as  if  the  flame  stood  behind  it.  The 
retina  proper,  on  the  other  hand,  reflects  less,  and  indeed  red  light. 
In  general,  the  image  of  the  flame  upon  it  appears  the  brighter,  the 
nearer;  the  darker,  the  farther,  it  lies  from  the  place  of  entrance  of 
the  optic  nerve.  On  the  contrary,  the  place  of  direct  vision,  the  yellow 
spot  (which  is  struck  when  the  observed  eye  D  gazes  directly  at  the 
mirrored  image  of  the  flame  at  B)  reflects,  by  way  of  exception,  very 
much  less  light  than  the  parts  which  are  nearest  to  it,  and  is  there- 
fore the  most  unfavorable  spot  for  this  experiment. 

In  order  to  fulfill  the  condition  that  the  observer  gaze  into  the  eye 


THE  OPHTHALMOSCOPE  13 

exactly  iu  the  direelion  of  the  iueideut  light,  the  glass  plate  may  be 
directed  either  by  the  observed  or  by  the  observer,  if  the  former  is 
to  do  it,  let  him  turu  the  plate  lirst  of  all  so  that  he  sees  therein  the 
mirrored  image  of  the  light,  then  again  so  that  this  image  appears  to 
him  exactly  iu  the  same  direction  as  the  observing  eye,  that,  iu  other 
words,  the  latter  and  the  mirrored  tiame  cover  each  other.  In  this 
way  the  required  condition  is  fulfilled.  At  the  same  time  occurs  this 
inconvenience,  that  the  observed  eye  must  look  directly  at  the  tiame, 
the  retinal  image  tlierefore  falling  precisely  on  the  spot  whence  the 
light  is  the  least  reflected.  If,  however,  the  observed  eye,  after  it  has 
found  the  correct  position,  turns  a  little  sidewise,  in  order  to  let  the 
light  shine  more  brightly,  then  the  pupil  becomes  displaced  and  the 
correct  position  is  disturbed.  Still,  one  can  then  assist  the  matter  by 
gentle  turning  of  the  mirror  now  this  way  and  now  that. 

It  is  better,  however,  to  perform  the  experiment  in  another  way, 
whereby  the  observer  holds  the  glass  himself.  One  must,  by  this 
method,  shade  the  face  that  is  being  observed,  and  make  the  reflecting 
plate  so  small  that  it  is  barely  large  enough  to  see  through.  The  light 
reflected  from  it  then  produces  on  the  shaded  face  of  the  observed  a 
small,  bright  spot,  which  has  about  the  form  of  the  reflecting  glass. 
This  point  should  be  so  managed  by  the  observer  that  its  centre  falls 
upon  the  observed  eye,  while  he  himself  looks  through  the  glass.  In 
this  way  the  glass  may  easily  be  placed  con-ectly,  and  the  observed 
eye  may,  without  the  slightest  difficulty,  be  turned  toward  all  sides 
in  order  to  cause  the  image  of  the  flame  to  fall  on  different  parts  of 
the  retina. 

Every  person  can,  furthermore,  in  similar  fashion,  by  the  aid  of  a 
bit  of  plane  glass,  see  one  of  his  own  eyes  grow  luminous.  He  should 
step  before  a  mirror,  set  up  a  lamp  at  one  side,  hold  the  glass  before 
his  right  eye  in  such  a  way  that  he  sees  the  flame  reflected  in  the  plate, 
and  turn  the  glass  so  that  the  image  of  the  flame  coincides  with  the 
mirrored  image  of  his  left  eye;  then  the  left  eye  sees  the  mirrored 
image  of  his  right  pupil  grow  luminous,  but  of  course  only  weakly  so, 
because  the  retinal  image  falls  on  the  outer  side  of  the  eye  at  a  con- 
siderable distance  from  the  optic  nerve. 

Moreover,  the  same  simple  expedient  permits  itself  to  be  employed 
with  advantage  for  illumination,  in  every  instance  when  one  wishes  to 
look  into  a  dark  cavity  with  a  narrow  opening,  for  example,  the  audi- 
tory meatus,  the  nose,  and  so  on.  In  order  to  view  the  drum  mem- 
brane, one  should  seat  the  subject  of  the  experiment  with  his  back 
toward  the  window,  preferably  in  sun.shine,  draw  the  auricle  a  little 
downward,  and  east  the  reflected  sunlight  into  the  auditory  meatus. 


14  THE  OPHTHALMOSCOPE 

while  one  gazes  through  the  glass.  In  this  way  one  may  very  easily 
and  conveniently  illuminate  the  tympanic  membrane  as  strongly  as  he 
wishes,  and  so  observe  it. 

In  order  to  see  the  pupil  become  luminous,  any  simple  plate  of  glass 
suliices  for  the  mirror ;  one  does  not  need  in  that  case  to  pay  particular 
attention  to  the  intensity  of  the  light.  Should  it  be  desired,  however, 
by  means  of  this  light  to  recognize  distinctly  the  structure  of  the  retina 
and  the  character  of  the  image  of  the  flame,  then  one  must  endeavor 
to  make  the  illumination  as  strong  as  possible.  That  can  be  done  in 
two  ways,  namely,  by  a  proper  choice  of  the  angle  under  which  the 
incident  light  is  reflected  from  the  mirroring  plane,  and  by  an  increase 
in  the  number  of  the  reflecting  plates.  I  will  now  unfold  the  prin- 
ciples which  have  guided  me  in  this  connection  during  the  construc- 
tion of  my  instrument,  and  which  would  also  serve  as  a  basis  should 
oculists  think  it  necessary  to  produce  modifications  of  the  instrument 
for  practical  purposes.  For  those  of  my  readers  to  whom  the  physical 
conceptions  involved  are  not  familiar,  I  remark  furthermore  that  this 
exposition  is  not  necessary  for  an  understanding  of  the  sections  to 
follow. 

From  every  limiting  surface  of  a  glass  plate,  the  more  light  is  re- 
flected the  larger  the  angle  of  incidence,  that  is,  the  angle  between  the 
ray  and  a  line  which  stands  vertical  to  the  plate.  Since,  in  the  ease  of 
reflection  from  the  upper  surfaces  of  transparent  bodies,  the  light 
waves  of  different  undulatory  directions  conduct  themselves  differ- 
ently, we  must  think  of  the  incident  light  as  divided  into  two  equal 
parts,  of  which  the  one  is  polarized  parallelly  to  the  reflecting  surface, 
the  other  vertically  thereto.  The  light-intensity  of  all  the  incident 
light  we  will  call  J,  therefore  that  of  each  of  the  two  divisions  men- 
tioned V2  J>  the  angle  of  incidence  (angle  between  the  incident  ray 
and  the  incident-perpendicular)  a,  the  angle  of  refraction  (between 
the  refracted  ray  and  the  incident-perpendicular)  aj,  the  index  of 
refraction  n.  If  a  is  given,  we  find  first  of  all  ai  by  means  of  the 
equation 

sin.  a  ^  n  sin.  a^. 

The  intensity  P  of  the  light  reflected  from  a  limiting  surface  between 
air  and  glass  and  polarized  vertically  to  the  plane  of  incidence,  is, 
according  to  the  formula  of  Fresnel 

J    tang2   (a  —  aj 
P  =  _. 

2     tang2   (a-f  0,) 


THE  OPHTHALMOSCOPE  15 

Likewise  the  intensity  Q  of  the  reflected  light  which  is  polarized  par- 

allelly  to  the  plane  of  incidence 

J     sin-   {a  —  flj) 

Q 


2     sin*   (a  -|-  oj 

When  several  reflecting  plates  lie  parallel,  one  behind  another,  and 
the  illuminating  surface  is  sufficiently  large  for  its  mirrored  images, 
which  are  produced  by  the  individual  reflecting  surfaces,  to  superim- 
pose themselves,  in  greatest  part,  for  the  observed  eye,  then  the  in- 
dividual images  combine  into  one  image  of  greater  brightness.  By 
computation  of  the  (luantities  of  light  reflected  to  and  fro  between 
the  different  surfaces,  one  is  able  to  determine  for  every  system  of 
parallel  surfaces,  how  much  light  is,  on  the  whole,  reflected.  For  an 
indefinite  number  n  of  the  reflecting  surfaces  one  finds  the  sum  11  of 
the  light  polarized  vertically  upon  the  plane  of  incidence 

nP 
n  = J 


J-l-2  (n  — 1)  P 

and  the  sum  ii  of  that  which  is  polarized  parallelly  to  the  plane  of 
incidence 

nQ 

S  = J 

J  +  2  (n  — 1)  Q 

As  I  find  these  formulae  in  no  writing  on  physics  I  give  their  deriva- 
tion briefly  at  the  end  of  this  essay. 

The  sum  n  -|-  2  gives  us  the  entire  quantity  of  light  which  is  thrown 
back  from  the  system  of  reflecting  surfaces  and  which  proceeds  to  the 
observed  eye.    We  will  set  it  down  as  equal  to  IT,  so  that 

H  =  n  +  5 

When  the  width  of  the  pupil  remains  unchanged,  the  brightness  of 
the  retinal  image  is  proportional  to  this  quantity  of  light.  The  quan- 
tity of  light  returning  from  the  eye  we  may  therefore  set  down  as 
equal  to  m  II,  where  m  designates  a  coefficient  whose  value  is  constant 
for  ditl'erent  light-intensities,  though  dependent  on  the  nature  of  the 
place  on  the  retina  from  which  the  light  proceeds.  The  returning  light 
divides  at  the  reflecting  surfaces  once  more  into  a  reflected  and  a 
transmitted  portion,  only  the  latter  arriving  in  the  observer's  eye. 
The  light  which  is  reflected  at  the  retina  possesses,  as  is  generally  the 


16  THE  OPHTHALMOSCOPE 

case  with  diffuse  reflected  light,  uo  longer  any  polarization,  conduct- 
ing itself  in  this  respect,  therefore,  like  the  light  from  the  light-source 
as  it  strikes  upon  the  mirror.  Inasmuch  as,  in  addition,  it  fails  upon 
the  plates  under  the  same  angle,  proportionately  as  much  of  it  is 
reflected  and  transmitted  as  of  the  former  [the  light  from  the  light- 
source].  If  we  designate  the  transmitted  part  by  X,  then  we  have 
the  proportion 

X  :  m  H  =  ( J  —  H)   :  J. 

From  this  may  be  computed  the  quantity  of  light  X,  which  passes  into 
the  eye  of  the  observer.  For  H  =  0  and  H  =  J ;  that  is,  when  no 
light  or  all  the  light  is  reflected,  X  will  =  0.  Between  these  extreme 
values  of  H  exists  a  maximum  of  the  value  of  X,  which  can  be  com- 
puted according  to  the  known  rules  of  the  differential  calculus.  The 
maximum  occurs  when 

H  =  i/o  J. 
Then  will 

X  ==  i/i  m  J. 

By  this  condition  is  also  determined  for  a  given  number  of  reflecting 
plates  the  angle  under  which  the  reflection  must  occur  in  order  to 
give  to  the  observer  the  brightest  image.  Unfortunately,  the  equation 
which  expresses  the  dependence  of  the  value  H  on  the  angle  of  inci- 
dence a,  cannot  be  solved  after  a;  we  can  therefore  find  the  proper 
values  of  a  only  approximately  by  means  of  computational  trials. 
Besides,  it  is  of  no  use  to  drive  the  exactness  of  this  computation  very 
far,  first,  because  the  brightness  for  the  observer  is  not  materially 
altered,  even  when  the  position  of  the  glasses  is  not  that  requisite  for 
the  maximum,  and,  secondly,  because  the  alterations  in  the  width  of 
the  pupil  pi'oduced  by  different  intensities  of  the  incident  light  can- 
not be  taken  into  account. 

As  the  pupil  of  the  observed  eye  becomes  smaller  under  stronger 
incident  light,  the  brightness  of  the  retinal  image  will  not  increase 
entirely  in  the  same  proportion,  when  the  values  of  H  increase,  as 
they  should  do  according  to  the  developed  formula?.  It  is  therefore 
more  advantageous  to  re-establish  in  the  instrument  the  values  of  H 
as  somewhat  smaller  than  would  be  requisite  for  the  maximum  of  H 
in  the  foregoing  computation.  One  reaches,  for  example,  the  value, 
which  slightly  deviates  from  the  foregoing  maximum, 

X  =  1/5  m  J 

when  the  light  is  reflected  from  one  glass  plate  at  an  angle  of  about 
70°,  from  three  at  an  angle  of  60°,  of  four  at  55°,  and  these  posi- 
tions are  therefore  approximately  the  most  advantageous. 


THE  OPHTHALMOSCOPE  17 

The  necessai-y  brightness,  therefore,  eau  eveu  be  reached  with  a 
SLUgle  glass  plate  for  a  mirror.  The  use  of  several  plates  at  a  smaller 
iuciaeiiL-e-aiigle  has,  however,  esseutial  advantages  if  oue  would  attaiu 
to  distiuet  images  of  the  retina.  1'  irst  of  all,  glass  plates,  eveu  when 
they  have  well  ground  parallel  surfaces,  are  not  always  internally  of 
so  homogeneous  a  structure  as  still  to  yield,  by  an  oblique  view,  good, 
distinct  images.  Then,  it  is  more  dilUcult,  by  a  very  oblique  view,  to 
give  to  a  reticcting  plate  the  correct  position  toward  the  observed  eye, 
and  to  hold  the  plate  therein.  Also,  the  observer,  by  the  lateral  parts 
of  his  head,  cuts  off  more  easily  the  rays  of  light  which  should  fall 
upon  the  mirror;  especially  may  this  be  avoided  with  difticulty  when 
the  angles  of  incidence  are  more  than  7U°.  Finally,  it  remains  to 
be  especially  considered  that  a  small  quantity  of  the  light  which  falls 
into  the  observed  eye  is  in  fact  i-eflected  from  its  cornea  and  appears 
to  the  observer  as  a  washed-out  light  spot  in  the  visual  held.  This 
falls  over  the  centre  of  the  pupil,  when  the  observed  eye  turns 
straight  toward  the  mirror,  therefore  when  it  looks  directly  at  the 
mirrored  image  of  the  flame ;  it  falls  more  to  oue  side  when  the  ob- 
served eye  gazes  in  any  other  direction,  disturbing,  however,  the 
observation  of  the  retina  always  more  or  less.  It  is  therefore  an 
essential  advantage  if  one  can  weaken  the  corneal  reflex  for  the 
observer  to  a  considerable  degree.  Now,  in  fact,  that  image  appears 
much  weaker  when  4  plates  reflect  at  56°,  than  when  3  reflect  at  60° 
or  one  at  70°,  while  the  retinal  image,  as  already  mentioned,  holds 
to  just  about  the  same  illumination.  That  is  to  say :  the  apparent 
brightness  of  the  corneal  reflex  is  not  proportional  to  that  of  the 
retinal  image,  because  the  light  which  falls  into  the  observed  eye,  and 
which  is  partly  or  wholly  polarized  by  reflection,  is  depolarized  by 
the  diffuse  reflection  at  the  retina — something  which  does  not  occur 
from  the  specular  reflection  at  the  cornea.  If  the  cornea,  of  the 
(|uantity  of  liglit  A  which  falls  upon  it,  reflects  the  portion  fxA,  then 
the  quantity  of  light  which,  in  our  experiments,  passes  from  the 
cornea  into  the  eye  of  the  observer,  equals,  according  to  the  same 
principles  and  the  same  designation  as  before. 


^ 


n  [J  —  2  n]  -f /Lt  S  [ J  —  2  2] 


Computation  gives  the  result  already  stated.  It  is  therefore  from 
every  point  of  view  more  advantageous  to  attain  the  necessary  bright- 
ness by  increasing  the  miniber  of  the  plates,  while  they  reflect  the 
light  at  the  polarization  angle  of  56°,  than  by  increasing  the  angle 


18  THE  OPHTHALMOSCOPE 

of  incidence,  indeed  the  corneal  reflex  could  be  made  to  disappear 
entirely  by  increasing  very  much  the  number  of  the  plates. 

1  have  assumed,  in  the  foregoing  explanations,  that  the  flame  of  a 
good  oil-lamp  with  a  double  draught  is  employed  as  the  light-source. 
When  the  experiment  is  properly  conducted,  the  light  of  such  a  lamp 
is  not  so  strongly  reflected  as  very  much  to  dazzle  or  fatigue  the 
lateral  parts  of  the  retina  of  the  observed  eye.  One  can  therefore 
easily  continue  the  experiments  as  long  as  one  likes.  Only  when  the 
eye  looks  directly  at  the  mirrored  image  of  the  flame,  will  this  degree 
of  brightness  be  found  not  long  endurable.  If  one  has  at  his  disposal 
a  more  intense  light,  for  example  sunlight,  which  falls  into  a  dark 
room  through  an  opening  in  the  window-shutter,  then  one  can  see  the 
picture  of  the  retina  much  brighter,  if  one,  after  proper  weakening 
of  the  light,  causes  it  to  reflect  from  a  mirroriug-plate  as  vertically 
as  possible,  than  when  this  takes  place  obliquely.  The  quantity  of 
light  which  one  may  permit  to  enter  into  the  eye  is  limited  particu- 
larly by  the  sensitiveness  of  the  lattei".  If,  now,  one  has  at  his  dis- 
posal excessively  strong  light,  which  by  every  kind  of  reflection,  if 
it  is  not  at  the  same  time  adequately  weakened  in  another  way,  ex- 
ceeds this  limit,  then  the  observer  sees  the  retinal  image,  which 
has  reached  the  limit  of  endurable  intensity,  at  its  brightest  when  as 
little  as  possible  is  lost  at  the  second  reflection.  That  is,  however, 
the  case  when  the  light  is  thrown  back  from  a  plate  almost  vertically. 

I  have  not  had  opportunity  to  institute  such  an  investigation  by 
means  of  sunlight;  I  do  not  believe,  however,  that,  by  that  method, 
any  considerable  advantages  are  to  be  secured,  because,  in  the  case 
of  vertical  reflection,  the  apparent  brightness  of  the  disturbing  cor- 
neal reflexes  increases  at  a  much  higher  rate  than  that  of  the  retinal 
image. 

There  was  expressed  to  me  a  number  of  times  the  supposition  (at 
first  blush  a  very  plausible  one)  that,  by  a  convex  lens  which  should 
concentrate  toward  the  observed  eye  all  the  light  which  falls  upon  it, 
the  quantity  of  light  falling  into  the  eye  and  therefore  also  the  bright- 
ness of  the  retinal  image,  could  be  considerably  increased.  I  will 
therefore  here  direct  attention  to  the  fact  that,  in  this  way,  not  the 
brightness  but  only  the  size  of  the  retinal  image  is  increased.  When 
we  bring  the  eye  to  the  point  of  union  of  the  light-rays,  which  have 
passed  through  a  lens,  then  the  entire  surface  of  the  lens  appears  to 
us  luminous  with  that  light-intensity  which  belongs  to  the  luminous 
point.  Instead  of  the  smaller  retinal  image  of  the  luminous  point, 
there  forms  itself  for  us  therefore  a  larger  one  with  the  same  in- 
tensity, that  of  the  lens-surface.     Moreover,  by  no  complicated  ar- 


THE  OPHTHALMOSCOPE  19 

rangement  of  lenses  can  the  biMghtness  be  increased.  In  order  to 
perceive  tliis,  we  need  only  to  remind  ourselves  of  this  fact  from 
the  theory  of  telescopes,  that  through  no  telescope  or  similar  ar- 
rangement of  lenses  can  an  object  of  appreciable  diameter  appear 
brighter  than  with  the  naked  eye.  As,  now,  the  inhabitant  of  the 
seeing  eye  subjectively  perceives  the  surface  no  brighter  through  the 
lenses,  so  can,  objectively,  the  image  in  his  eye  by  the  use  of  no  sort 
of  lenses  be  brighter  than  without  them.  For  to  an  objectively 
brighter  retinal  image  there  must  always  correspond  a  stronger  sub- 
jective light-perception. 

2.  Production  of  a  Distinct  Image  of  the  Retina. 

We  now  come  to  investigate  how,  by  means  of  the  light  which, 
returning  from  the  retina  of  the  observed  eye,  falls  into  the  eye  of 
the  observer,  we  may  be  able  to  receive  distinct  images  of  the  retina 
itself,  and  of  the  picture  of  the  light-source  cast  upon  it.  For  this 
purpose  let  us  take  again  our  Fig.  1.  According  to  the  explanations 
just  made,  the  ocular  media  will  so  refract  the  rays  returning  from 
points  of  the  retina  of  the  eye  D,  that  they  come  together  outside  the 
eye  and  indeed  in  the  corresponding  points  of  the  image  B.  The 
image  which  the  ocular  media  cast  of  the  retina  and  of  the  retinal 
image  of  the  flame,  coincides  therefore  in  size  and  position  with  the 
first  reflected  image  of  the  flame.  An  observer  who  (reckoning  out- 
ward from  the  mirror)  stands  on  the  other  side  of  B  and  at  the 
distance  of  distinct  vision  from  B,  would  therefore  in  fact  be  able  to 
see  that  image  of  retinal  objects  distinctly.  His  visual  field,  how- 
ever, limited  by  the  pupil  of  the  observed  eye,  would,  at  the  com- 
paratively considerable  distance  of  the  two  eyes  from  one  another,  be 
so  small  that  it  would  be  impossible  to  combine  the  viewed  details 
into  a  complete  picture. 

The  regard  which  we  must  pay  to  the  enlargement  of  the  visual 
field,  makes  it  much  more  necessary  to  approximate  the  two  eyes  as 
closely  to  each  other  as  possil)le.  Then,  however,  the  image  B  falls 
in  general  behind  the  back  of  the  observer,  and  can  not  be  plainly 
seen  by  him.  If,  for  example,  in  Fig.  1,  the  observing  eye  is  at  G, 
then  it  receives  the  light  rays  which  proceed  out  of  the  eye  D  and 
which  come  together  at  the  points  of  B.  Now  a  normal  eye  can  in- 
deed unite  upon  its  retina  parallel  rays,  as  these  move  from  infinity, 
and  divergent,  as  these  come  from  nearer  points,  but  not  convergent 
rays.  The  simplest  way  to  assist  in  this  matter,  and  to  make  the 
convergent  bundles  of  rays  divergent,  is  a  concave  lens,  which  is 


20  THE  OPHTHALMOSCOPE 

inserted  between  the  mirror  and  the  eye  of  the  beholder,  as  in  Fig. 
1  at  F. 

According  to  the  known  laws  of  refraction  in  concave  lenses,  the 
convergent  rays  which  strike  upon  F  will,  after  their  exit  from  the 
lens,  either  be  less  convergent  (when,  that  is  to  say,  the  focal  distance 
is  greater  than  FB)  or  they  become  parallel  (when  the  focal  distance 
equals  FB)  or,  finally,  divergent,  as  if  they  came  from  points  of  an 
image  E  behiud  the  obsei-ved  eye  (when  the  focal  distance  is  smaller 
than  BF).  In  the  latter  ease  the  concave  lens  acts  precisely  as  it  does 
in  opera  glasses,  where  it  likewise  converts  the  inverted  imperfect 
image,  which  the  objective  lens  should  cast  at  its  focus,  and  which 
lies  on  the  side  of  the  observer,  into  one  which  stands  upright  and 
which  appears  to  the  observer  to  be  on  the  other  side  of  the  glasses. 
In  our  case,  likewise,  the  ocular  media  form  the  objective  glass  of  a 
microscope,  which  is  constructed  on  the  principle  of  a  Gallileonian 
telescope,  while  the  concave  lens  represents  the  ocular. 

If  the  accommodation  distance  of  the  two  eyes  DB  and  GE  are 
given,  and  in  addition  the  mutual  distances  of  the  eyes  and  the  con- 
cave lens  are  settled  according  to  the  principles  above  set  forth,  that 
is,  made  as  small  as  the  mirror  permits,  then  is  the  focal  distance 
which  is  to  be  given  to  the  concave  lens  to  be  determined  according 
to  the  known  laws  of  refraction  in  lenses.    This  is  found  to  equal 

EF    BF 


EB 
or: 

(EG  — GF)    (BD  — DF) 


EG  +  BD  — DG 


The  greater  are  the  accommodation  distances  EG  and  BD,  the 
greater  must  also  be  the  focal  distance  of  F.  The  observer  will, 
therefore,  if  one  of  the  two  eyes  is  short-sighted,  employ  stronger  con- 
cave lenses,  but,  if  one  eye  is  far-sighted,  weaker  ones,  than  for  two 
normal  eyes.  When  the  observing  and  the  observed  eye  exchange 
their  roles,  without  altering  the  condition  of  their  accommodation, 
there  will  generally  become  necessary  a  glass  of  a  different  focal  dis- 
tance, and,  indeed,  as  GF  <  DF,  a  weaker  one,  when  the  more  short- 
sighted eye  observes,  than  when  it  is  observed.  Still,  a  closer  con- 
sideration of  the  foregoing  formula  shows  that  this  difference  is 
extremely  slight  in  the  case  of  not  too  short-sighted  eyes,  so  that,  in 
the  case  of  such,  the  same  glass  can  serve  for  mutual  observation. 


THE  OPHTHALMOSCOPE  21 

The  magnification  is  determined  according  to  the  known  laws  of 
optics  in  this  way,  that  the  image  E,  viewed  from  the  center  of  the 
lens  F,  must  appear  under  the  same  visual  angle  as  B,  its  imaginary 
object.  Since  the  eye  G,  the  lens  F  and  the  eye  D  stand  as  closely 
together  as  possible,  then  will  B  appear  from  F  only  a  little  larger 
than  from  D.  The  eye  G  therefore  sees  the  retinal  image  of  the  flame 
magnified,  and  indeed  just  as  large,  or,  considered  exactly,  a  trifle 
larger,  than  the  eye  D  sees  the  original  flame.  The  parts  of  the 
retina  on  which  the  image  of  the  flame  falls,  appear  likewise  in  the 
image  E  again,  magnified  of  course  in  the  same  proportion  as  that. 

According  to  what  has  just  been  said,  the  proportion  of  this  en- 
largement is  equal  to  that  of  the  retinal  image  to  its  object.  Let  us 
take  as  the  distance  of  the  decussation-point  of  the  refracted  rays 
from  the  retina,  according  to  Volkmann's  measurings,  4  lines,  for  the 
distance  of  the  object  from  the  eye  the  normal  visual  distance  of  8 
inches,  then  the  magnification  is  found  to  be  24  times. 

We  have  compared  the  ocular  media  in  our  experiment  with  the 
objective  of  a  microscope,  the  concave  glass  with  the  ocular.  Now,  in 
place  of  the  latter,  one  should  be  able  to  produce  a  combination  of 
two  convex  lenses,  which  stand  at  a  distance  from  one  another  of 
less  than  the  sum  of  their  focal  distances,  as  is  the  ease  in  the  ordi- 
nary compound  microscope.  The  first  of  the  lenses  would,  like  the 
collective  glass  of  this  instrument,  unite  the  weakly  converging  light- 
rays  which  proceed  from  the  observed  eye,  more  promptly  to  an 
image,  which,  situated  between  the  lens  and  its  focal  distance,  would 
exhibit  the  flame-image  upright,  the  retina  inverted.  This  image 
could  then  be  seen  magnified  by  the  second  convex  lens.  I  have 
debated  the  results  of  such  a  combination,  according  to  the  known 
laws  of  optical  instruments,  with  respect  to  magnification,  illumina- 
tion, visual  field,  etc.  As  the  computation  showed  that  in  this  way 
no  essential  advantages  were  to  be  secured,  as  compared  with  the 
simple  concave  lenses,  it  will  here  suffice  to  adduce  those  results  very 
briefly.  It  is  hereby  presupposed  that  the  first  lens,  so  far  as  the 
mirror  permits,  is  approximated  to  the  observed  eye,  and  that  the 
observing  eye  lies  close  to  the  second  lens. 

First  of  all,  as  to  the  illumination,  the  maximum  thereof  is  directly 
attained  by  a  concave  lens  for  the  middle  of  the  visual  field.  If  the 
same  thing  is  to  occur  by  two  convex  lenses,  then  these  must  be  so 
chosen  and  arranged  that  no  other  enlargement  takes  place  than  by 
the  concave  lens,  that  is,  in  such  a  way  that  the  magnified  retinal 
image   of  the  flame  appears  to  the  observing  eye  under  the  same 


22  THE  OPHTHALMOSCOPE 

visual  angle  as  the  mirrored  image  of  the  flame  does  to  the  eye  that 
is  being  observed. 

If  this  enlargement  is  to  occur,  the  image  from  the  first  lens  must 
fall,  as  in  the  ordinai-y  ocular  tubes  of  the  compound  microscope,  in 
the  middle  between  both  lenses.  In  the  ease  of  a  weaker  magnifica- 
tion, it  is  possible  to  cause  a  larger  portion  of  the  visual  field  to 
appear  in  the  maximum  of  brightness;  in  the  case  of  stronger,  on 
the  contrary,  that  can  no  longer  occur  even  in  the  middle.  As  ad- 
vantageous, therefore,  as  even  a  stronger  magnification  might  be,  still 
such  a  one  is  not  practicable,  because  the  illumination  would  thereby 
suffer  too  much,  and  a  living  eye  would  not  well  endure  for  a  longer 
time  without  dazzling  the  incidence  of  still  stronger  light  than  that 
reflected  from  a  good  lamp.  Then,  too,  is  the  fact  that  the  living  e_ve 
cannot  be  thoroughly  fastened,  as  would  be  necessary  for  the  fixation 
of  individual  parts  of  the  image  in  the  case  of  stronger  magnification. 
Next  to  be  considered  is  the  visual  field.  The  part  of  the  retina 
which  one  can  survey  is  always  the  smaller  the  farther  one  removes 
oneself  from  the  observed  eye;  the  larger  the  nearer  one  comes  to  it. 
The  limit  of  approximation  is,  however,  set  in  this  way :  that  the 
obliquely  placed  mirror-plates  have  to  be  inserted  between  the  eye 
and  the  glass-lenses. 

In  order  to  compare  by  means  of  computation  the  effects  of  various 
lenses,  we  must  therefore  accept  as  equallj-  great  the  distance  of  the 
concave  glass  and  that  of  the  first  convex  glass  from  the  observed 
eye.  If  then  at  the  same  time  the  condition  is  observed,  that  the 
brightness  in  the  middle  of  the  visual  field  should  reach  its  maximum, 
then  are  found  definite  focal  distances  of  the  convex  lenses  for  every 
given  distance  from  the  eye,  which  make  the  visual  field  at  its  largest. 
If  one  choose  the  focal  distances  of  both  the  convex  lenses  in  accord- 
ance with  these  determinations,  then  it  further  appears  that  when 
the  distance  of  the  lens  from  the  ej'e  is  smaller  than  the  focal  distance 
which  one  may  give  to  the  objective  of  a  telescope  from  the  aperture 
of  the  pupil  without  prejudicing  the  distinctness  of  the  image,  there- 
fore in  the  case  of  achromatic  lenses  smaller  than  perhaps  the  ten- 
fold pupillary  diameter,  the  concave  lens,  if  larger,  the  convex  lenses 
can  give  a  larger  visual  field.*  Now,  in  the  case  of  the  closest  possible 
approximation  of  the  lenses  to  the  obsei*\'ed  eye,  the  distance  between 
both  will  of  course,  on  account  of  the  mirror  being  placed  in  the 


*  The  senteripe,  in  the  original,  is  hopelessly  obscure;  it  is,  therefore,  also 
obscure  in  the  translation.  The  reader  should  recall  the  fact  that  Helmhnltz.  at 
the  time  when  he  wrote  the  "  Beschreibwrg,"  was  not  yet  master  of  a  literary 
stvic,  and  T  liave  deemed  it  far  the  fairer  way  not  to  force  into  the  sentence  a 
meaning  of  my  own. —  (T.  H.  S.) 


THE  OPHTHALMOSCOPE  23 

interval  between  the  lenses,  remain  in  general  somewhat  larger  than 
the  tenfold  pupillary  diametei",  and  one  would  therefore  be  able  to 
secure  by  means  of  two  convex  lenses  a  slight  advantage  for  the  visual 
field.  Inasmuch,  however,  as  the  lenses,  in  order  to  yield  this  ad- 
vantage, must  have  focal  distances  of  36  to  -10  lines,  it  may  become 
very  diHicult  to  receive  an  image  of  the  same  distinctness  as  by  a 
concave  lens  which  may  have  a  focal  distance  of  8  to  10  inches.  I, 
at  least,  have  not  been  successful  in  this  matter,  by  the  combination 
of  such  convex  lenses  as  stood  at  my  disposal.  Moreover,  it  transpii-ed, 
in  the  experiments  with  such  lenses,  that  the  correct  location  of  the 
instrument  for  the  perception  of  the  retinal  image  is  both  found 
and  kept  with  much  greater  difficulty.  With  a  simple  concave  lens 
it  is,  to  wit,  not  necessary  that  the  axis  of  the  lens  be  directed  exactly 
upon  the  observed  eye,  if  only  the  mirror  casts  light  into  it.  This  con- 
dition, however,  must  be  observed  in  the  case  of  two  convex  lenses. 

Consec]ueutly  it  appears  to  be  more  advantageous  to  retain  the 
simple  concave  lens  as  ocular,  while  one  almost  everywhere  else  in 
optics  replaces  it  to  decided  advantage  by  convex  lenses.  A  decided 
advantage  of  the  latter  occurs,  to  be  sure,  even  in  our  case,  which 
would  make  their  emploj'ment  desirable,  to  wit,  the  advantage  that, 
by  an  altered  distance  of  the  lenses  from  each  other,  one  can  adjust 
the  apparatus  to  all  visual  distances  of  the  observed  and  the  observ- 
ing eye,  while,  for  this  purpose,  one  must  exchange  the  concave  lens 
for  another.  If  one  could  completely  make  fast  the  head  of  the 
observed  person  and  tlie  instrument,  convex  lenses  would  in  conse- 
quence be  more  convenient ;  without  such  arrangements,  however,  all 
their  other  advantages  are  outweighed  by  the  disadvantage  of  the 
difficult  placing  of  the  instrument.  I  have  therefore  myself  always 
employed  only  a  simple  concave  lens. 

3.  Description  of  the  Opiitii.vlmoscope. 

In  order  to  institute  observations  of  the  kind  described,  it  is  con- 
venient to  unite  tlie  mirror-plates  and  the  concave  lens  by  means  of 
a  suitable  frame.  I  propose  for  such  a  combination  the  name  Atigen- 
spiegel,  by  analogy  with  similar  instruments.  The  instrument  is 
viewed  in  Fig.  2  from  in  front,  in  Fig.  3  exhibited  in  horizontal 
cross-section.  The  reflecting  plates  hh  are  fastened,  by  means  of  the 
brass  piece  gg,  to  the  circular  plate  aa,  at  an  angle  which  is  equal  to 
tiie  chosen  angle  of  incidence  of  the  light  rays — in  the  figure,  56°. 
The  brass  piece  gg  forms  with  the  glass  plates  a  hollow,  right-angu- 
larly triangular  prism.    In  Fig.  3  one  sees  into  the  inner  cavity  there- 


24  THE  OPHTHALMOSCOPE 

of,  and  lias  before  him  one  of  the  riglit-angularly  triangular  basal  sur- 
faces. Of  the  three  quadi'augular  lateral  surfaces  of  the  prism,  that 
which  corresponds  to  the  hypotheuuse  of  the  basal  surtace,  is  formed 
by  the  glass  plates,  that  which  corresponds  to  the  longer  cathetus 
stands  free,  that  corresponding  to  the  shorter  cathetus  lies  on  the  disc 
aa,  and  carries  a  cylindrical  process  p,  which,  by  means  of  a  corre- 
sponding circular  opening  in  the  plate  aa,  so  clasps  through,  that  it 
holds  the  prism  fast  against  the  plate,  but  permits  a  turuiug  on  its 
axis.  The  glass  plates  are  held  against  the  prismatic  brass  piece  by 
the  frames  kkkk,  wliose  over-reaching  lateral  edges  are  secured  to  the 
brass  piece  gg  by  the  screws  11.  The  disc  aa  rests  on  the  cylinder  bbec 
without  being  permanently  fastened  to  it.  In  the  border  of  aa,  namely, 
there  are  cut  four  openings  of  the  form  f,  to  which  openings  there 
correspond  four  screws  ee  with  cylindrical  heads  and  thin  necks, 
inserted  into  the  border  of  the  cylindrical  ring  bb.  in  Fig.  2  are 
shown  only  two  of  these  screws,  in  order  to  let  the  holes  f  be  seen. 
The  heads  of  the  screws  allow  of  their  shoving  through  the  broad 
circular  portions  of  the  openings,  and  if  then  the  disc  aa  is  turned 
about  its  center,  the  necks  of  the  screws  pass  into  the  smaller,  slit- 
shaped  part  of  the  same  opening,  while  their  heads  lap  over  and 
fasten  the  disc  to  the  ring  bb.  In  that  way  it  is  possible  to  remove 
the  disc  very  easily  and  quickly  from  the  setting  of  the  concave  lens, 
and  to  exchange  the  lens  for  another.  The  concave  lens  nu  lies  be- 
tween the  plate  aa  and  the  floor  of  the  cylindrical  piece  dd,  which  is 
screwed  into  bbcc  and  can  be  set  back  by  screwing  round,  when  it 
becomes  necessary  to  lay  two  lenses  one  upon  the  other  for  very  short- 
sighted eyes.  The  whole  is  fastened  to  the  handle  m.  For  a  normal- 
eyed  observer,  the  numbers  6  to  12  of  the  ordinary  concave  spectacle 
lenses,  are  suiScient  for  the  adjustment  to  all  adaptational  conditions 
of  the  eyes  to  be  investigated.  For  the  viewing  of  other  normal  eyes, 
I  generally  employed  Nr.  10.  For  very  short-sighted  eyes,  two  lenses 
should  be  superimposed. 

As  to  the  reflecting  plates,  those  of  ordinary  mirror-glass  are  not 
appropriate,  because  their  two  surfaces  are  as  a  rule  not  sufficient!}' 
parallel  to  cause  the  images  which  they  cast  of  the  lamp-flame  to 
coincide  in  the  way  that  they  should.  The  glasses  must  therefore  for 
our  use  be  especially  ground,  in  order  to  receive  parallel  surfaces, 
though  this  condition  need  not  be  fulfilled  with  such  exactness  as  in 
the  case  of  the  plane-parallel  glasses  which  one  employs  in  the  finer 
measuring  instruments. 

A  good  blackening  of  the  non-reflecting  surfaces  is  essential.  Since, 
of  the  bright  light  which  falls  upon  the  instrument,  only  a  propor- 


THE  OPHTHALMOSCOPE  25 

tionately  small  part  returns  from  the  retina  of  the  observed  eye,  all 
the  remaiuiug  remnants  of  the  light,  which  might  perhaps  get  into 
the  eye  of  the  observer,  must  be  done  away  with.  First  of  all,  the 
inner  surface  of  the  ocular  piece  dd  must  be  blackened,  and  the  ob- 
server must  place  his  eye  as  closely  iuto  it  as  possible,  iu  order  to  cut 
off  all  the  light  which  could  fall  from  the  flame  upon  this  surface. 
Secondly,  the  outer  surface  of  the  disc  aa  and  of  the  prismatic  mirror- 
frame  kkkk  must  be  blackened,  iu  order  that  blank  metal  surfaces, 
which  are  turned  toward  the  observed  eye,  may  not  produce  disturb- 
ing corneal  reflexes.  Thirdly,  however,  the  inner  surface  of  the 
mirror-frame  is  to  be  blackened  with  especial  care.  The  light  of  the 
flame  which  falls  on  the  reflecting  plate,  passes  in  greater  part  through, 
and  strikes  the  plate  gg.  All  that  is  not  here  absorbed,  returns  to  the 
mirror,  is  reflected  from  this  in  the  same  direction  to  the  observing 
eye,  in  which  the  weak  light  from  the  retina  of  the  observed  eye 
arrives,  and  mingles  with  the  image  of  this  membrane.  I  have  found, 
in  this  matter,  the  general  methods  of  procedure  of  mechanics  for 
blackening  brass-pieces  to  be  inadequate,  and  the  framework  of  the 
mirror  must  be  tapestried  internally  with  black  velvet,  which  ab- 
sorbs the  light  more  completely.* 


*Tho  subsequent  history  of  the  ophthalmoscope  "down  to  a  time  within  the 
memory  of  men  still  living,"  is,  very  lirielly,  as  follows;  Ruete,  in  1852,  invented 
the  "indirect  method"  (D.  Av{ienspie<jcl  u.  d.  Optometer,  Giittingen,  1852).  He 
employed  a  concave  perforated  mirror,  held  at  a  considerable  distance  from  the 
ol  served  eye,  and  letwcen  the  mirror  and  the  eye  one  (sometimes  two)  spherical 
convex  lenses.  Ilelmholtz,  also,  had  made  use  of  convex  lenses,  but  these  he  had 
placed  behind  the  mirror,  finding  them  there,  of  course,  of  very  little  value  (see 
herein). 

Helmholtz,  next,  explained  most  thoroughly  (Vierordt's  Archiv,  1852,  p.  827) 
the  method  which  Ruete  had  invented.  Tn  the  very  same  paper  Helmholtz  described 
what  he  called  "the  simplest  method,"  by  which  an  eye  could  be  examined  by 
means  of  only  a  candle,  a  screen  and  a  convex  spherical  lens.  He  also  mentioned 
(still  in  the  same  most  momoraljle  article)  the  so-called  Rekoss  discs — i.  e.,  two 
rotatory  discs,  each  containing  four  concave  lenses  inserted  not  far  from  the 
perijiheries  of  the  discs.  One  of  the  discs  held  lenses  from  fi  in.  to  9  in.  focus, 
the  other  those  from  10  to  13  in.  The  Rekoss  disc,  or  discs,  with  numerous  modi- 
fications, is,  as  all  are  aware,  in  use  at  the  present  day.  Rekoss  was  not  an  oidithal- 
mologist,  but  an  instrument  maker  of  Konigsl.erg  (where  Helmholtz  at  the  time 
was  living). 

Coccius,  in  185.3,  invented  an  instrument  which  found  much  favor  for  years. 
It  consisteil  of  a  lens,  set  in  a  frame,  in  front  of  a  plane  mirror.  The  distance 
between  the  mirror  anii  the  lens  could  bo  altered  very  considerably. 

Kduard  Jaeger,  in  1854,  produced  a  combination  of  the  Helmlioltz  and  the 
Ruete  instrument — that  is  to  say,  the  plates  of  silvered  glass  in  the  Helmholtz 
instrument  were  made  replaceable  by  a  concave  silvered  mirror,  such  mirror  to  be 
u^ed  for  the  indirect  method.  To  this  affair  of  Jaeger's,  Strawbridge,  of  Phila- 
delphia, in    1871,  added   three  interchangeable  Rekoss  discs. 

The  ophthalmoscope  of  I.iebreich  is  too  familiar  to  all  to  require  the  slightest 
description.  So,  almost,  is  that  of  Loring,  with  its  single  disc  and  double  row  of 
lenses,  the  disc  being  movable  up  and  down  for  the  jnirposo  of  bringing  into 
action   either  the  one   row   or   the   other.     Wadsworth,   of   Boston,   invented   the 


26  THE  OPHTHALMOSCOPE 

When  one  desires  to  use  the  instrument,  he  sets  the  person  to  be 
examined  in  a  dark  room  and  next  the  corner  of  a  table  on  which,  at  a 
level  with  the  eye  and  sidewise  from  the  face,  stands  a  well-burning, 
double-draught  lamp.  It  is  convenient  to  set  upon  the  table,  at  a 
fitting  visual  distance,  some  not  too  bright  object,  whereon  one  can 
point  out  to  the  observed  eye  certain  points  for  lixation,  for  example 
a  blackboard  divided  into  squares,  each  of  which  is  designated  by  a 
number,  while  one  causes  the  eye  to  fix  various  points  one  after  an- 
other. The  image  of  the  flame  falls  ever  on  different  parts  of  the 
retina,  which  the  observer,  therefore,  may  investigate  one  after  an- 
other in  any  order  desired.  Between  the  flame  and  the  observed  eye 
an  opaque  screen  must  be  erected,  in  order  to  shade  the  eye,  so  that 
directly  incident  flame-light  may  not  produce  a  very  disturbing  cor- 
neal reflex  and  a  narrowing  of  the  pupil.  Still,  the  border  of  the 
shadow  must  pass  very  close  before  the  observed  eye,  in  order  that 
the  ophthalmoscope,  which  must  itself  remain  in  the  light,  may  be 
carried  toward  that  eye  as  closely  as  possible.  The  observer  seats 
himself  before  the  observed,  brings  the  ophthalmoscope,  without  at 
first  looking  through  it,  into  about  the  right  position,  when  its  reflect- 
ing surface  casts  a  bright  light  upon  the  face.  When  one  has  so  turned 
the  mirror  that  the  middle  of  its  light  falls  upon  the  eye,  and  the 
axis  of  the  instrument  is  directed  precisely  into  it,  one  looks  through. 
A  person  then  has,  as  a  rule,  at  once  before  him  the  bright  image  of 
the  flame,  or  finds  it  after  more  or  less  moving  about.  Moreover,  one 
can  also,  looking  through  the  instrument,  discern,  to  a  certain  extent, 
the  eye  and  the  clear  light  which  must  fall  upon  it,  even  if  indis- 
tinctly and  as  if  they  were  faded,  and  also,  in  that  manner,  with  the 
help  of  these  [the  eye  and  the  light  upon  it]  discover  the  correct  posi- 
tion. If,  though  the  pupil  appears  luminous,  one  cannot  see  the  various 
parts  of  the  retina  distinctly,  then  one  must  insert  another  concave 
lens.  An  observer  who  has  accustomed  himself  to  alter  at  will  the 
adaptation  of  his  eye,  easily  discovers  whether  he  sees  more  plainly 
by  a  far-sighted  or  a  near-sighted  adaptation,  and  whether,  accord- 
ingly, he  must  choose  more  or  less  strongly  curved  lenses.  Moreover, 
many  persons  make  the  matter  difficult,  especially  those  who  are  not 
accustomed  to  looking  through  optical  instruments,  and  short-sighted 
persons  who  see  through  them  with  difficulty,  insomuch  as  they  invol- 
untarily adapt  the  eye  for  great  nearness,  because  they  think  of  the 


"mirror  obliniiolv  set,"  which  enables  the  nbserrer  today  to  look  straight  through 
the  lenses  instead  of  at  an  an<Tle.  Both  the  Loring  and  the  Wadsworth  instru- 
ments are  especially  valuable  for  refraction  purposes. 

The  electric  light  ophthalmoscopes  are  in  the  hands  of  every  practising  oph- 
thalmologist at  the  present  day,  and  require  no  description. 


THE  OPHTHALMOSCOPE  27 

object  to  be  seen  as  beiug  very  close.  In  that  way  the  eyes  of  the  ob- 
server are  greatly  fatigued,  and  readily  begin  to  be  injected  and  to 
water.  It  is  necessary  here,  as  in  the  case  of  all  optical  instruments 
possessing  an  alterable  adaptation,  to  adjust  the  eye  for  the  distance, 
and  then  to  adjust  the  instrument  to  the  eye. 

After  a  little  practice  it  is  not  difficult  to  find  the  right  lens  and 
the  correct  position  of  the  instrument.  Also,  one  can  easily,  on  his 
own  eye,  show  these  matters  to  anyone  who  has  never  yet  seen  them,  in 
order  first  to  render  him  familiar  with  the  appearance  of  what  he  is 
going  to  look  for.  In  that  way  it  will  be  made  much  easier  for  him 
to  discover  independently  the  very  same  things  in  the  eyes  of  others. 
Let  the  instructor,  for  this  purpose,  first  of  all  discover  the  particular 
lens  through  wliich  he  can  see  the  student's  retina  plainly,  and  then 
let  him  place  this  in  the  ophthalmoscope;  then  through  the  same  glass 
the  student  can  see  distinctly  into  the  eye  of  the  teacher,  if  neither  of 
the  two  is  very  short-sighted.  In  the  latter  case  (as  explained  al- 
ready) the  more  short-sighted  person  needs  a  somewhat  weaker  glass 
when  he  observes  than  when  he  is  observed.  Let  the  instructor,  then, 
bring  one  of  his  own  eyes  into  the  position  which  has  been  described 
as  that  for  the  eye  to  be  observed,  and  let  him  so  hold  the  ophthal- 
moscope before  him  that  he  may  be  able,  at  the  same  time,  to  look 
through  its  central  openings  and  glimpse  the  mirrored  image  of  the 
flame  in  the  mirror,  hand  over  to  the  student  the  instrument  in  this 
position,  and  let  him  look  through  it.  The  student  will  then  see  in 
the  eye  the  image  of  the  flame.  In  order  to  teach  him  to  recognize 
the  appearance  of  the  parts  of  the  retina,  let  the  teacher  throw  the 
image  of  the  flame  on  the  place  of  entrance  of  the  optic  nerve,  beeaiise 
in  that  place  the  largest  and  most  recognizable  vascular  trunks  exhibit 
themselves.  Let  him,  for  this  purpose,  turn  the  eye  gradually  more 
and  more  to  the  inner  side  of  the  mirrored  image  of  the  flame,  until  this 
suddenly  becomes  smaller  to  him,  or  disappears.  That  happens,  as 
is  known,  when  the  image  falls  upon  the  place  of  entrance  of  the  optic 
nerve.  Besides,  most  persons  more  easily  succeed  in  seeing  and  recog- 
nizing the  imacre  of  the  flame  than  the  tiny  parts  of  the  retina  in  the 
bright  ground  thereof. 

4.  Viewing  the  Retina  and  the  Image  op  the  Flame. 

Should  one  desire  to  investigate  the  retina  completely,  then  it  is 
convenient,  as  already  mentioned,  to  set  up  a  blackboard  covered 
with  numbers  as  a  visual  [loint  for  the  eye  to  be  investigated.  As 
soon  as  this  eye  fixes  one  of  the  numbers,  looking  past  the  mirror  a 


28  THE  OPHTHALMOSCOPE 

little  to  the  inward  side  thereof,  the  observer  will  almost  always  recog- 
nize in  the  visual  held  one  or  two  of  the  larger  vessels.  He  causes  the 
eye  to  turn  to  one  of  the  near-lying  figures,  and  netices  whether  he  is 
brought  nearer  to  the  origin  or  to  the  branching  of  the  vessels.  While, 
in  this  way,  he  traces  the  vessels  in  the  direction  of  their  larger  trunks, 
he  comes  at  length  to  the  place  of  entrance  of  the  optic  nerve.  This 
distinguishes  itself  from  the  rest  of  the  eye-ground  by  its  white  color, 
for  it  is  not  covered  with  pigment  and  a  hne  vascular  network,  but 
here  the  white  cross-section  of  the  nerve  lies  wholly  free,  at  the  very 
most  shot  through  by  tiny,  isolated  vessels.  Mostly  to  the  inner 
side,  near  by,  the  arteries  and  veins  of  the  retina  press  forward  from 
the  depths.  At  times  one  sees  a  portion  of  the  vessel  still  hiding  in 
the  substance  of  the  nerve,  and  understands  that,  in  the  living,  this 
substance  is  decidedly  transparent.  One  distinguishes  the  two  kinds 
of  vessels  from  each  other  by  the  brighter  color  of  the  blood  and  the 
double  contours  of  the  walls  in  the  arteries  and  in  their  first  ramifica- 
tions. I  have  not  been  able  to  recognize  pulsations  with  certainty. 
The  first  main  branches  of  the  vessels  border  the  optic  nerve  at  its 
inner  side,  in  order  to  spread  out  later,  above  and  below,  across  the 
retinal  field.  The  appearance  of  the  sharply  pencilled  red  vessels  on 
the  clear  white  ground  is  of  surprising  elegance.  Somewhat  farther 
to  the  inner  side,  close  by  the  nerve,  I  have  always  remarked  a  small, 
sickle-shaped  stripe  of  shadow,  which  appears  to  take  its  origin  from 
a  fold  of  the  retina. 

In  the  other  parts,  the  ground  of  the  eye  looks  reddish,  and  in- 
deed first  of  all  round  about  the  optic  nerve  of  a  somewhat 
clear,  light-red,  the  darker,  on  the  contrary,  the  farther  you 
pass  from  that  place.  One  sees  here  larger  and  smaller  branch- 
ing blood-red  vessels,  which  stand  out  plainly  from  the  back-ground. 
The  ground  itself  appears  to  be  not  entirely  homogeneous,  but  indis- 
tinctly reddish.  This  would  seem  to  arise  from  the  fact  that  the  close 
capillary  net  is  too  fine,  too  weakly  illuminated  and  too  transparent 
to  be  distinguished  plainly  from  the  underlying  weakly,  light-gray 
substance  of  the  retina.  That  the  ground  looks  brighter  in  the  vicin- 
ity of  the  optic  nerve  is  no  doubt  owing  to  the  fact  that  the  retina 
here,  on  account  of  the  superimposed  fibres  of  the  optic  nerve,  is 
thicker,  while,  toward  its  periphery,  it  becomes  continually  thinner. 
]\Ioreover,  the  place  of  direct  vision  (the  yellow  spot)  is  essentially 
distinguished  in  appearance  from  the  parts  which  lie  immediately 
aboiit  it.  In  order  to  got  this  point  before  oneself,  one  causes  the 
eye  which  is  being  observed  to  look  directly  at  the  mirrored  image  of 
the   flame.      The   retina  then   appears  much  darker,   grayish-yellow 


I 
i 


THE  OPHTHALMOSCOPE  29 

without  intermixture  of  I'cd;  and  one  sees  no  traces  of  capillary  ves- 
sels. Then,  too,  one  is  greatly  annoyed  while  gazing  on  the  yellow 
spot,  by  the  tiny  image  from  the  cornea,  which  obtrudes  itself  pre- 
cisely in  the  center  of  the  visual  field,  while,  during  the  observation 
of  the  lateral  portions  of  the  retina,  it  lies  to  one  side. 

After  deciding  what,  in  the  healthy  eye,  can  be  made  out  concern- 
ing the  nature  of  the  retina,  I  have  no  doubt  that  one  will  be  able 
to  recognize  all  such  disease  conditions  as  permit  of  recognition  by 
the  sense  of  sight  in  other  transparent  parts — for  example,  the  cornea. 
Increased  repletion  of  the  vessels  and  vascular  varicosities  must  prove 
easy  to  make  out.  Exudates  into  the  substance  of  the  retina,  or  be- 
tween that  structure  and  the  pigment  membrane,  must  yield  them- 
selves to  observation,  very  much  as  affections  of  the  cornea  do,  by 
their  brightness  against  a  dark  ground.  If  they  lie  in  part  before 
the  retina,  they  will  then  enclose  its  vessels  in  a  veil.  I  here  recall 
that,  according  to  Briicke,  the  recent  retina  is  just  about  as  trans- 
parent as  the  other  ocular  media,  and  that,  apart  from  its  vessels,  it 
is  only  visible  in  our  experiments  because  it  is  strongly  illuminated 
on  the  deep-black  ground  of  the  pigment  membrane.  Fibrinous 
exudates,  which  are  nearly  always  less  transparent  than  the  ocular 
media,  must  also  for  that  reason,  when  they  lie  in  the  fundus  of  the 
eye,  considerably  strengthen  the  reflex.  Then  too  I  believe  thnt  opaci- 
ties of  the  vitreous  body  will  be  much  more  easily  and  certainly  recog- 
nizable, partly  by  the  illumination  of  a  reflecting  glass-plate,  partly 
by  the  ophthalmoscope.  One  will  even  be  able  to  determine  with  ease, 
from  the  indistinctness  of  the  image  of  the  flame  and  of  the  retinal 
vessels,  the  degree  of  the  opacity.  If,  in  the  case  of  such  an  opacity, 
scintillating  particles  have  detached  themselves,  then  too  a  person  will 
be  able  to  take  note  of  these.  In  brief,  I  believe  that  I  may  hold  the 
expectation  not  to  be  exaggerated,  that  all  the  alterations  of  the 
vitreous  body  and  of  the  retina  which,  until  now,  have  been  found  in 
cadavers,  will  also  permit  of  recognition  in  the  living  eye — a  possi- 
bility which  appears  to  promise  the  most  remarkable  advances  for  the 
hitherto  undeveloped  pathology  of  this  structure.* 

Finally,  it  is  of  interest,  for  certain  physiological  purposes,  to  in- 
vestigate the  accuracy  with  which  the  eye  forms  images.  It  is  best  to 
employ  for  this  purpose  a  thread,  which  one  draws  along  horizon- 


•PrnbaMy  the  most  sicmifioant  sentence  ever  ■pennP'I  hv  an  orhtlislmolo'T'st. 
How  crIo''i''iislv  the  tre^t  man's  nrorhecv  has  been  fnlflllpfl  is  Vnnwn  nnt  merely 
to  B^ef'ialists  and  cenera]  prartitionerp,  but  even,  in  ponip  rle'rree.  to  first  venr 
ine'1i''.il  st"'1eTit»  nnd  the  efli'i'aterl  portion  of  the  laitv.  In  fnft.  therp  are  i'lst 
twn  kinrlq  of  ophthnlmoloary,  that  whii'h  came  before  ari'l  thit  which  followed  after 
Helmholtz's  " Beschreibung  eines  Augenspiegds." — (T.  H.  S.) 


30  THE  OPHTHALMOSCOPE 

tally  before  the  flame.  Its  image  remains  single,  while  vertical  threads 
are  manifolded  by  the  manifold  reflections. 

First  of  all  one  gets  an  opportunity  to  convince  oneself,  by  the 
appearance  of  the  image,  that  the  difllerent  adaptations  of  the  eye 
really  depend  upon  alterations  in  the  refractive  media.  One  should 
cause  to  be  fixed  an  object  which  is  just  about  as  far  removed  from  the 
observed  eye  as  the  thread  is  from  the  flame.  The  observer  then  sees 
the  elements  of  the  retina  and  the  image  of  the  thread  distinctly  at  the 
same  time.  Should  the  thread  be  carried  nearer  to  or  farther  from 
the  eye,  then  it  becomes  indistinct  in  the  retinal  image,  or  entirely 
disappears,  while  the  parts  of  the  retina  remain  sharp.  One  perceives 
from  this  that  the  retinal  images  of  objects  which  stand  at  various 
distances  from  the  eye,  are  in  fact  not  equally  distinct.  Then  again, 
one  should  so  place  the  thread  that  it  appears  distinct  in  the  retinal 
image  at  the  same  time  with  the  vessels,  and  should  cause  the  observed 
eye  to  fix  a  point  which  is  either  much  farther  or  much  nearer  than 
that  upon  which  it  was  formerly  directed.  Immediately  one  sees 
the  retina  and  the  image  of  the  flame  become  gradually  indistinct. 

It  should  incidentally  be  remarked  that,  on  the  white  surface  of 
the  optic  nerve  no  image  is  cast,  even  when  the  image  appears  abso- 
lutely sharp  on  the  immediately  surrounding  portions  of  the  retina. 
Inasmuch  as  the  observer,  in  the  case  of  a  person  over  whose  optic 
nerve  cross-section  little  vessels  run,  sees  these  quite  as  plainly  as 
those  of  the  retina  adjacent,  therefore  that  indistinctness  of  the  image 
of  the  flame  cannot  proceed  from  the  passage  of  the  end  of  the  optic 
nerve  out  of  the  level  of  the  retina.  I  believe  rather  that  one  must 
regard  the  transparent  condition  of  the  optic  nerve  mass  as  the  real 
cause. 

Moreover,  one  is  able,  whenever  it  becomes  necessary,  to  convince 
oneself  readily  in  an  objective  manner  of  the  presence  and  the  degree 
of  the  short-  or  far-sightedness  of  the  observed  eye.  Let  the  observer 
first  investigate  a  normal  eye,  which  he  causes  to  fix  objects  at  vari- 
ous distances,  and  notice  what  concave  lenses  he  is  obliged  to  use  in 
the  various  stages  of  adaptation  of  the  eye.  In  the  investigation  of 
any  other  eye,  he  then  learns  from  the  number  of  the  concave  glass 
through  which  he  saw  the  retina  distinctly  the  corresponding  adapta- 
tional  distance  of  the  observed  eye.  The  observer  is,  by  this  method, 
entirely  independent  of  the  assertions  of  the  other  person,  for  he 
himself  sees,  as  it  were  with  that  other's  eye,  at  least  by  means  of  its 
refractive  media.  In  this  way,  for  example,  I  was  able  to  convince 
myself  in  a  completely  amaurotic  eye,  that  that  eye  was  simultane- 
ously in  a  high  degree  short-sighted.     In  that  way  was  decided  in 


i 


THE  OPHTHALMOSCOPE  31 

this  case  a  question  of  great  importance  for  the  anamnesis,  whether, 
that  is,  certain  earlier  dituculties  of  sight  recounted  by  the  patient, 
should  be  referred  to  shortsightedness  or  to  comiiiencing  amblyopia. 

An  important  physiological  conclusion  thrust  itself  upon  me  in 
these  investigations,  'the  iree-lying  cross-section  of  the  optic  nerve  is 
apparently  so  transparent  that  the  light  which  falls  upon  it  must 
penetrate  deeply  into  the  mass  of  the  nbres,  inasmuch  as,  now  and 
then,  one  sees  the  bendings  of  the  central  artery  and  vein  shimmering 
forward  through  the  substance  of  the  nerve.  When  the  little  image  of 
the  Mame  falls  on  the  place  of  entrance  of  the  nerve,  then  all  its  nbres, 
or  at  least  a  very  large  part  of  them,  are  struck  by  more  or  less  in- 
tense light,  and  yet,  obviously,  they  perceive  no  light.  If  they  did 
perceive  it,  then  that  entire  portion  of  the  visual  field  which  cor- 
responds to  them  would  have  to  appear  illuminated.  Not  only,  how- 
ever, is  that  not  the  case,  but  there  is  even  less  light  perceived  than 
when  the  image  falls  upon  some  other  portion  of  the  retina.  We  must 
from  this  conclude  that  the  fibres  of  the  optic  nerve  are  incapable 
of  being  affected  by  objective  light  (ethereal  vibrations),  while,  never- 
theless, they  perceive  every  other  kind  of  irritation  as  subjective  light. 
This  is  a  paradox,  which,  of  course,  has  its  ground  in  the  ambiguity 
of  the  word  "light,"  and  is  far  removed  from  being  an  actual  con- 
tradiction. The  vibrations  of  the  ether  which  we  call  light,  produce, 
like  every  other  mechanical  or  electrical  irritation,  when  they  strike 
the  retina,  the  sensation  which  we  call  light.  But  from  this,  that  the 
retina,  protected  from  pressure  and  electrical  currents  and  exposed  to 
the  action  of  ethereal  vibrations,  is  much  ofteuer  struck  and  excited 
by  the  former  than  by  the  latter,  it  by  no  means  follows  that  light 
must  be  regarded  as  an  especially  adequate  irritant  for  the  retina 
and  the  elements  of  the  optic  nerve  and  as  standing  in  contrast  to  all 
the  other  kinds  of  irritation.  There  are  no  difSculties  in  supposing 
that  all  the  irritations  which  are  able  to  affect  the  optic  nerve  system 
produce  sensations  of  light,  that,  however,  the  ethereal  vibrations  are 
able  to  act  only  on  the  retina.  A  similar  state  of  affairs  is  found  in  the 
case  of  the  nerves  of  touch,  with  respect  to  heat  and  cold.  Here  too 
the  peripheral  expansions  behave  differently  from  the  trunks.  For 
the  latter,  slight  variations  in  temperature  are  no  irritant  at  all,  as 
it  appears,  and  the  greater  variations,  which  are  able  to  irritate,  pro- 
duce no  temperatural  sensations.  Besides,  one  is  able  to  conclude 
still  further  that,  in  the  retina,  not  the  fibres,  which  spread  out  in  a 
radiating  manner  from  the  optic  nerve,  but  the  spherical  elements, 
are  sensitive  to  light.  "Were  it  the  former,  then  must  light  which 
strikes  on  any  place  in  the  retina  be  perceived  by  all  those  fibres  which 


32  THE  OPHTHALMOSCOPE 

in  part  end  in  this  place,  and  in  part  pass  across  it  on  their  way  to- 
ward the  retinal  periphery.  There  would  therefore  extend,  in  the 
visual  field,  from  every  illuminated  point,  a  bright  shine  toward  the 
borders  of  the  field,  which  is  not  the  case.  We  maj'  accordingly  fur- 
ther conclude  that  even  the  continuations  of  the  optic  nerve  fibres  in 
the  retina  are  insensitive  to  light.  There  remain  only  the  ganglionic 
bodies  and  the  nuclear-like  structures  of  the  retina,  in  which  the 
ethereal  oscillations  are  able  to  act  as  an  irritant. 

Appendix. 

Derivation  of  the  formula  for  the  quantitij  of  light  which  is  re- 
flected from  several  glass  plates. 

Whether  this  formula  is  correct  for  n  reflecting  surfaces  is  shown 
by  the  fact  that  it  is  also  correct  for  (n  +  1).  As  it  also  proves  right 
for  n  =  1  and  n  =  2,  it  must  do  the  same  for  any  desired  value  of  n. 

Let  the  quantity  of  light  which  at  the  given  angle  of  incidence  is 
thrown  back  by  a  reflecting  surface,  when  the  quantity  1  passes  off  of 
light  polarized  vertically  against  the  plane  of  incidence,  be  p,  that 
thrown  back  by  n  such  surfaces  P,„),  that  thrown  back  by  (n-f-1), 
P(nti).    It  is  demonstrable  that  if 

np 

P.n,= 1) 

l+(n-lp) 
then  also  that  equation  is  correct  which  arises  from  this  by  the  sub- 
stitution of  n  -|-  1  for  n : 

(n  +  l)p 

1  +np 

For  the  sake  of  a  better  designation,  let  us  assume  that  the  system 
of  n  reflecting  plates  lies  horizontal  and  that  light  falls  on  it  from 
above.  Let  the  (n  -|-  1)  th  surface  be  added  to  the  system  below.  The 
quantity  of  light  which  passes  downward  from  the  lowermost  nth 
surface  of  the  compound  system  to  the  (n  +  1)  th  surface  let  us  call 
X;  that  which,  reflected  from  the  (n  +  1)  th  surface,  mounts  to  the 
system  of  the  n  surfaces,  y.  The  quantity  x  is  composed  partly  of  the 
portion  of  the  incident  light  which  has  passed  through  the  system 
of  n  surfaces,  partly  of  the  portion  of  y  which  is  reflected  from  this 
system.     Therefore  is 

x  =  l-P,„,+yP,„, 3) 


THE  OPHTHALMOSCOPE  33 

The  quantitj'  y  originates  from  that  pai"t  of  x  which  is  reflected  from 
the  (u  +  l)th  surface.    It  is  therefore 

y  =  xp   4) 

Tlie  quantity  P(„+i)  which  passes  upward  from  the  uppermost  sur- 
face, proceeds  in  part  from  that  portion  of  the  incident  light  which 
is  reflected  from  the  system  of  n  surfaces,  partly'  from  that  portion 
of  y  which  passes  through  this  system.    It  is  therefore 

P(n,l,  =  P,n,  +  y     (1— P(n,) 5) 

If  one  eliminates  x  and  y  from  equations  3,  4  and  5,  one  gets 

P[l-P(n,]- 

P(Dti)  =  "(n)  H  6) 

1  — PP(0> 

If  we  place  in  this  equation  6  the  value  of  P(n,  from  equation  1,  we 
get  in  fact,  after  the  necessary  reductions,  equation  2,  whose  cor- 
rectness was  to  be  proved. 
For  one  reflecting  surface  is 

P<i)=P 
Equation  one  (to  be  tested)  gives  the  same  value. 

For  two  reflecting  surfaces  we  get  the  value  P,,,  without  employ- 
ing equations  1  or  2,  if,  in  the  derivation  of  equation  6,  we  suppose 
that  n  =  l  and  (n)P^p.     Equation  6  then  becomes 

P(l-P)- 

P<=)=P  + 

l-p2 

2p 


1  +  P 
Equation  1  gives  the  same  value. 

As  the  latter  accordingly  is  correct  for  n  ^  1  and  for  n  =  2,  then 
it  follows  from  the  proof  adduced,  that  it  is  correct  also  for  n  =  3,  and 
if  it  is  correct  for  n  ^  3,  that  it  also  is  correct  for  n  =  4,  and  so  on 
to  infinit}'. 

In  a  precisely  similar  way  the  matter  proceeds  in  the  case  of  light 
polarized  parallel  to  the  surface  of  incidence. 

If  we  assume  the  quaiititv  of  incident  light  to  be  equal  to  Y-t  J,  and 
2P, 

that  p  = and  designate  that  which  we  have  here  called  P  with 

J 
II,  we  get  the  formula  in  question. 


UC  SOUTHERN  REGIONAL  LIBRARY  FACILITY 


D    000  780  780    3 


